This invention relates to electrical devices such as fuses and lightning arrestors and, more particularly, to enclosures for such electrical devices which are particularly well suited to an underground, or generally submersible, installation, or similar installations where the conditions ambient the enclosed electrical device may vary.
In its on-line mode of operation the enclosed fuse, or lightning arrester, is at line potential and the voltage stress in the area of the fuse must be controlled to minimize the occurrence of corona and it well known deleterious effects. Corona shields carried by a housing enclosing a fuse have been used in the past, examples of such arrangements can be found in U.S. Pat. Nos. 3,588,607, 3,686,604, and 3,818,407. Similarly, corona shields carried by the actual fuse housing have been used, and examples can be found in U.S. Pat. Nos. 2,593,426 and 2,844,691. The structures shown in these patents are only typical and are not to be considered exhaustive of the prior art arrangements which have been proposed.
This invention is concerned with this problem of controlling voltage stress to minimize the occurrence of corona and has as one of its general objects the provision of an improved corona shield arrangement for an enclosed electrical device such as a fuse or lightning arrester.
An additional object is to optimize corona prevention while reducing the overall size of the enclosed electrical device and while remaining consistent with design parameters for an underground, or generally submersible, electrical installation.
Another specific object of this invention is to insure that all electrical stress occurs across the insulation material of the factory molded housing and not across air spaces normally occuring in the chamber defined by the enclosure.
It is believed that this invention will find perhaps its widest and most immediate application in connection with enclosed non-expulsion type fuses such as sand current-limiting fuses and vacuum fuses. Therefore, it will be discussed in that enviroment but should not be interpreted as being so limited in its application; as already stated, it may be used with lightning arresters as well as other electrical circuit interrupter devices. In such applications, deposits due to ambient conditions may occur naturally on the interior of the housing enclosure as the unit stands on line and, even in such applications, some gases may evolve during interruption even though such fuses are basically non-expulsion. Such gas evolution may contribute to the deposits. It is recognized that these deposits can form a conductive path between terminals and should be avoided. Also, internal pressures sufficient to rupture the enclosure are to be avoided. This invention is also concerned with these problems and further objects of this invention are to provide, in the housing interior, an effective barrier to the completion of this conductive path and to prevent excessive internal pressure buildup.